There are many more threads on this problem but no solutions. This is not an isolated problem, but I get the feeling NVIDIA doesn't realize the scope of the problem. If you've experienced this problem, please post relevant information so we can get NVIDIA to fix this problem once and for all.

Disabling AGP didn't make a difference.
Not enabling fast writes didn't make a difference.
Not enabling SBA didn't make a difference.
Not using framebuffer console didn't make a difference.
Disabling ACPI with acpi=off didn't make a difference.
Disabling APIC with noapic didn't make a difference.
Setting pci=noacpi didn't make a difference.
Setting pci=biosirq didn't make a difference.

It's definitely not a hardware problem. I don't get random segfaults, which would indicate a hardware problem. I can run Quake 3 and Unreal Tournament 2004 for several hours without problems, which wouldn't be possible with bad hardware. I've tested the memory with memtest86 and found no problems. The video card isn't sharing an IRQ with any other device.

The system doesn't really crash. X simply stops responding. The system continues to run in the background. This problem does not occur with the open source "nv" driver.

After dealing with this problem for several months, I finally figured out a pattern. The problem only seems to occur if I run X for several days, then restart it. The "lockup" then occurs within 24 hours or so. I don't know if this is the case with everyone else, but perhaps that might be a clue as to why it happens.